It turns out that it's a port of one of my favoured PC Engine games: Override. It's basically the same game: same ship, same weapons, same enemies, same controls...
But it's a whole lot different. For one thing, the game almost ten times larger, requiring two 1.2MB floppies compared to the PCEngine's 2Mbit HuCard. It now has a lengthy introduction sequence, new levels, new enemy and player sprites, new player animations, new powerup sprites, etc etc etc. It's a complete overhaul, and even the game has received some changes: the purple weapon's completely gone, and there's a new back-fire charge attack.
I'm not convinced it's as good as the PC Engine version. There's something lacking that I have never been able to put my finger on, and so this rediscovery has a goal: figure out whether it's as good, and find out why.
This article is part of a series of retro gaming rediscoveries. You can view the rest of them right here.